1. Field of the Invention
The present invention relates to a semiconductor device and method for manufacturing the same using a non-hermetic package such as a mold package, and in particular to a semiconductor device and method for manufacturing the same which can prevent deterioration of radiofrequency characteristics and improve the moisture resistance.
2. Background Art
Schemes to generalize radiofrequency semiconductor devices such as field effect transistors using a chemical compound semiconductor such as GaAs or GaN are rapidly progressing and there is a strong demand for a cost reduction effect. As a means for meeting this demand, using low-priced mold packages instead of completely hermetic metal packages heretofore used has become practical. In a case where a non-hermetic package such as a mold package is used for a semiconductor device, however, it is necessary to improve the moisture resistance of the semiconductor device in order to prevent deterioration in various respects due to causes relating to water.
Conventionally, electrodes provided on a main surface of a semiconductor substrate are covered with thick insulating film such as SiN film formed by plasma CVD or the like, thereby preventing permeation of water and securing the moisture resistance.
The existence of the thick insulating film having a high dielectric constant between the semiconductor substrate and the electrodes, however, entails a problem that the capacitive components are increased and the radiofrequency characteristics are deteriorated. Insulating film formed by plasma CVD or the like easily absorbs moisture, depending on the film forming condition. If the film thickness is increased, the insulating film can be separated by a change in stress when it absorbs a small amount of water. In such case, the coverage and film quality at a stepped portion of a transistor are deteriorated to allow water to easily pass through or to be absorbed in the film. It is, therefore, difficult to effectively prevent permeation of water to the transistor.
A semiconductor device has been proposed that has a construction for preventing deterioration of radiofrequency characteristics in which an air gap is formed between a main surface of a semiconductor substrate and an air gap forming film; the air gap envelops gate electrodes and drain electrodes; and an opening of the air gap is closed with a resin (see, for example, Japanese Patent Laid-Open No. 2009-184067, particularly FIGS. 30 and 31).
In Japanese Patent Laid-Open No. 2009-184067, the method of connecting the electrodes in the air gap to electrode pads outside the air gap is not described in detail. In a case where the electrodes and the electrode pads are connected to each other by metal wirings, an end of each metal wiring is connected to the electrode and the other end of the metal wiring is led out of the resin to be connected to the electrode pad. However, a gap can occur easily at the interface between the metal wiring and the resin when the resin is thermally cured in the manufacturing process. As a result, the moisture resistance is deteriorated.